定义一个哺乳动物Mammal类，再由此派生出狗Dog类，二者都定义 Speak()成员函数，基类中定义为虚函数，定义一个Dog类的对象，调用Speak函数，观察运行结果

class Mammal

{

public:
Mammal(){cout<<"Mammal"<<endl;}
virtual void speak(){}
virtual ~Mammal()
{
cout<<"~Mammal::speak()"<<endl;
}

};

class Dog:public Mammal

{

public:
Dog(){cout<<"Dog"<<endl;}
virtual void speak()
{
cout<<"Dog::speak"<<endl;
}
~Dog()
{
cout<<"~Dog::speak()"<<endl;
}

};

void main()

{
Mammal *p;
p=new Dog;
p->speak();
//(*p).speak();
delete p;
p=NULL;
}

运行结果：



如若基类的析构函数不写关键字virtual，则只会调用基类的析构函数，不会调用派生类的析构函数